Climate change is making trees bigger, but weaker

As global temperatures rise, trees around the world are experiencing longer growing seasons, sometimes as much as three extra weeks a year. All that time helps trees grow faster. But a study of the forests of Central Europe suggests the higher temperatures—combined with pollution from auto exhaust and farms—are making wood weaker, resulting in trees that break more easily and lumber that is less durable.

“I worry that wood may not be as strong as it used to be,” says Richard Houghton, an ecologist at the Woods Hole Research Center in Falmouth, Massachusetts, who was not part of the new study. He says the findings mean that forests may suffer more damage from storms and may be less efficient at soaking up the greenhouse gas carbon dioxide (CO2) than scientists had thought.

For the past 100 years, trees have been experiencing growth spurts in temperate regions from Maryland to Finland, to Central Europe, where the growth rate of beech and spruce has sped up nearly 77% since 1870. Assuming wood is just as dense today, those gains would mean more timber for building, burning, and storing carbon captured from the atmosphere.

But Hans Pretzsch, a forest scientist at the Technical University of Munich in Germany, and his colleagues wondered whether the quality of the wood has changed. To check, they started with 41 experimental plots in southern Germany, some of which have been continuously monitored since 1870. Pretzch and his team took core samples from the trees—which included Norway spruce (Picea abies), sessile oak (Quercus petraea), European beech (Fagus sylvatica), and Scots pine (Pinus sylvestris)—and analyzed the tree rings using a high-frequency probe.

They found that in all four species, wood density has decreased by 8% to 12%, they report online in Forest Ecology and Management. “We expected a trend of the wood density like this, but not [such] a strong and significant decrease,” Pretzsch says. Increasing temperatures, and the faster growth they spur, probably account for some of the drop. But another factor, Pretzsch says, is more nitrogen in the soil from agricultural fertilizer and vehicle exhaust. Previous studies have linked increased fertilizer use to decreased wood density.

As the density of the samples dropped, so did their carbon content, by about 50%. That means the trees have been sucking up less CO2 from the atmosphere every year, Houghton says. But Pekka Kauppi, an environmental scientist at the University of Helsinki, says another factor more than makes up for the climate impact: the faster-growing forests. “The negative change of wood density is by far less important than the positive change” of a global turnaround from shrinking to expanding forests, he says.